It is well-known in the art that certain materials called phosphors can be irradiated with high energy ionizing radiation, and then subsequently stimulated to produce an emission. Thermoluminescent phosphors are currently in widespread use in radiation dosimeters used to measure the amount of incident radiation to which people, animals, plants and other things are exposed. Thermoluminescent dosimeters are widely used by workers in the nuclear industries to provide a constant monitor for measuring exposure to radiation.
Phosphors are excited by energetic radiation such as ultraviolet, X-ray, gamma, and other forms of radiation. Such ionizing radiation causes electrons within the thermoluminescent material to become highly energized. The nature of thermoluminescent materials causes these high energy electrons to be trapped at relatively stable higher energy levels. The electrons stay at these higher energy levels until additional energy, usually in the form of heat, is supplied which releases the trapped electrons, thereby allowing them to fall back to a lower energy state. The return of the electrons to a lower energy state causes a release of energy primarily in the form of visible light which is ordinarily termed a luminescent emission.
The use of thermoluminescent phosphors in personnel dosimeters has led to demand for a large number of dosimeters which must be read on a routine basis in order to monitor exposure of persons or other objects to ionizing radiation. Because of the substantial numbers and the relatively slow reading techniques currently employed, the job of reading dosimeters becomes very time consuming and costly.
There are four commonly known methods of heating thermoluminescent material in order to release the trapped electrons and provide the luminescent emission which is measured as an indication of the amount of ionizing radiation to which the dosimeter was exposed. The first and most common method for heating thermoluminescent phosphors is by contact heating. The second method is heating using a hot gas stream which is impinged upon the phosphor. The third method uses radiant energy in the form of infrared beams which heat the thermoluminescent phosphor. The fourth method uses infrared laser beams to provide the necessary heat for luminescent emission.
Novel methods and apparatuses for laser reading of thermoluminescent phosphor dosimeters are disclosed in detail in U.S. Pat. Nos. 4,638,163 and 4,839,518 incorporated by reference hereinabove. One of the inventors of this invention and his colleagues have developed laser reading techniques and dosimeters, as disclosed in an article entitled "Laser Heating In Thermoluminescence Dosimetry," by J. Gasiot, P. Braunlich, and J. P. Fillard, Journal of Applied Physics, Vol. 53, No. 7, July 1982. In that article, the authors describe how thin layers of thermoluminescent phosphors can be precipitated onto glass microscope cover slides and used as laser readable dosimeters. Powder layers of the phosphors were in some cases coated with a thin film of high temperature polymers. The content of said article is hereby incorporated hereinto by reference.
Laser heating of thermoluminescent phosphors is superior because of the greatly decreased heating times and associated increased processing rates which are possible. Release of stored luminescent energy within a short period of time greatly improves signal-to-noise ratios and thus the accuracy of dosimeter measurements.
It is desirable in the monitoring of radiation dosage to discriminate between different types of radiation. In the case of some types of radiation it is desirable or necessary to use specific forms of dosimeters for detecting and measuring that type of radiation as distinct from other types of dosimeters used for detecting other forms of radiation. For example, the measurement of gamma radiation can be accomplished using a single thermoluminescent dosimeter element which is heated uniformly to a desired temperature thus causing a thermoluminescent emission to occur. The thermoluminescent emission is measured and the resulting luminescence is interpreted and has been found useful as an accurate measurement of the amount of ionizing radiation to which the dosimeter was exposed. Alternatively, it is possible to heat a large number of small localized areas of a dosimeter to identify the approximate number of localized areas which have been ionized by the impingement of a heavy charge particle. The proportion of areas which have experienced such a heavy charged particle event can used as an indication of the amount of heavy particle radiation to which a dosimeter has been exposed. Such radiation dose measurement techniques are appropriate for radiations such as alpha particles and neutrons among others. The methodology of such dose measurement techniques is further explained in the incorporated by reference parent application Ser. No. 336,015.
In addition to the heat stimulation of phosphors it is also possible to stimulate them with laser beams in a phenomenon call optically stimulated luminescence. In optically stimulated luminescence the laser beam is directed in an intense beam having high power for very brief periods of time. This form of laser stimulation is explained in U.S. Pat. No. 4,507,562 which is hereby incorporated by reference.
In light of these differing approaches for measuring gamma radiation versus heavy particle radiation, and other differing laser dosimeter reading techniques, it has not been practical to include dosimeters on a single badge having differing stimulating laser beam requirements and accordingly some forms of radiation have not been monitored. It has further not been possible to use a single laser dosimeter reader to stimulate radiation dosimeters having distinct beam requirements. Accordingly, there has been a need in the art for laser dosimeter reading equipment which can read multiple types of radiation dosimeters having differing stimulating beam requirements using a single laser source.